1. Field of the Invention
The present invention relates to a semiconductor device having a plurality of semiconductor elements and a method for manufacturing the same.
2. Description of the Related Art
When providing a plurality of semiconductor elements over an insulating surface, a method is employed in which a semiconductor film formed over an insulating surface is processed into a plurality of island-shape semiconductor layers by etching. A semiconductor element has a stacked structure of a plurality of thin films. In the case of a planar thin film transistor, a gate insulating layer is stacked so as to cover semiconductor layers which are isolated in island shapes.
Semiconductor layers which have been processed into island shapes have steps at the ends of the layers; therefore, a gate insulating layer formed over the ends of the semiconductor layers could have defects such as a thin film thickness or breaking of the film.
When a thin gate insulating layer is formed, characteristic defects of a semiconductor device could be caused such as a leakage current which flows between a gate electrode and a semiconductor layer. Further, if the gate insulating layer breaks, the gate electrode and the semiconductor layer could have a contact and thus be shorted.
In order to solve the foregoing problems, a method has been known in which two gate insulating layers with different shapes are stacked, so that steps resulting from the ends of the semiconductor layers are mitigated and thus the coverage of the semiconductor layers is improved (e.g., see Reference 1: Japanese Published Patent Application No. H10-242471).